Japanese Laid-Open Patent Application No. 9-312609 discloses a method of controlling transmission power in a soft handover process of a mobile station that performs radio communication with a plurality of base stations in a CDMA mobile communication system.
In accordance with this transmission power control method, each of the two base stations that are to receive signals from the mobile station measures the signal-to-interference plus noise power ratio (hereinafter referred to as the “reception SINR”, and generates transmission power control bits (each of which represents a power increase or a power decrease) so that the reception SINR approaches a target SINR. The transmission power control bits are then transmitted from each base station to the mobile station. The mobile station measures the reception SINR with respect to each signal transmitted from the two base stations. Regarding the reception SINR of each base station as the reliability level of each transmission power control bit transmitted from the corresponding base station, the mobile station performs a transmission power control operation (a power increasing operation, a power decreasing operation, or a power maintenance operation) based on the transmission power control bits transmitted from each base station.
More specifically, if the reception SINR of one of the base stations is equal to or lower than a predetermined reference level, the transmission power control bits transmitted from the base station are regarded as unreliable and therefore are ignored. Instead, a transmission power control operation (a power increasing operation or a power decreasing operation) is performed based on the transmission power control bits transmitted from the other base station. If the reception SINRs of both base stations are equal to or lower than the predetermined reference level, the transmission power control bits from both base stations are regarded as unreliable, and the current transmission power is maintained regardless of those transmission power control bits. If the reception SINRs of both base stations are higher than the predetermined reference level, a transmission power control operation is performed based on such transmission power control bits as to decrease the transmission power.
By the above transmission power control method, a transmission power control operation is performed at the mobile station in such a manner that the transmission power is reduced based on the transmission power control bits having the higher reliabilities among the transmission power control bits transmitted from each base station. Thus, efficient transmission power control can be performed while a reasonable communication quality level is maintained.
The mobile station performs maximum-rate combining on the reception SINRs measured with respect to the base stations to obtain a composite SINR. Based on the composite SINR, the mobile station generates and transmits transmission power control bits to each base station. Based on the transmission power control bits transmitted from the mobile station, each base station performs a transmission power control operation.
In the above transmission power control operation at a mobile station in a mobile communication system, the reception SINR of each base station is regarded as reliable, based on the transmission power control bits transmitted from the corresponding base station. However, the reception SINR is measured at the mobile station based on a signal transmitted from each base station also performing such a transmission power control operation as to maintain the signal reception quality (the composite SINR) at a certain level at the mobile station. Because of this, the reception SINR does not necessarily reflect the true conditions (such as the fading condition and the distance) of the electric wave transmission path between the mobile station and each base station. Particularly, in a soft handover process, the conditions of the electric wave transmission path between the moving mobile station and each base station constantly vary with time. Therefore, it is preferable to take into consideration the true conditions of the electric wave transmission path in a transmission power control operation at the mobile station. In this manner, the transmission power at the mobile station can be reduced, while the predetermined communication quality is maintained.
Conventionally, a mobile station and a base station that perform radio transmission and reception of signals with each other in a CDMA mobile communication system each control the transmission power in the following manner.
The mobile station measures the signal-to-interference plus noise power ratio (hereinafter referred to as the “reception SINR”) with respect to each reception signal from the base station, and regards the reception SINR as the reception signal quality. Based on the difference between the reception SINR and a target SINR, the mobile station generates transmission power control bits (each of which represents a power increase or a power decrease). The mobile station then transmits the transmission power control bits (transmission power control information) to the base station.
The base station controls the transmission power based on the transmission power control bits transmitted from the mobile station, while measuring the reception SINR with respect to each reception signal from the mobile station. Based on the difference between the reception SINR and a target SINR, the base station generates transmission power control bits, and transmits the transmission power control bits to the mobile station.
The mobile station generates the transmission power control bits to be transmitted to the base station, as described above, while controlling the transmission power at the mobile station based on the transmission power control bits transmitted from the base station.
By the above transmission power control operations at the mobile station and the base station, the transmission power at the mobile station is controlled so that the reception SINR at the base station approaches the target SINR. Also, the transmission power at the base station is controlled so that the reception SINR at the mobile station approaches the target SINR. Even if there are changes in the conditions (such as the distance and the fading condition) of the electric wave transmission path between the mobile station and the base station, the mobile station and the base station can perform signal transmission and reception with a stable reception signal quality, but without unnecessary transmission power consumption, by the above transmission power control operations.
When the above transmission power control operations are properly performed at the mobile station and the base station, each reception SINR at the mobile station and the base station fluctuates within a relatively narrow range, with the target SINR being in the middle, which is shown as the “NORMAL” region in FIG. 20. However, even when the above transmission power control operations are properly performed, the reception SINR might continuously decrease and move away from the target SINR, as shown in the “ABNORMAL” region of FIG. 20. The reasons of the continuous decrease of reception SINR are considered as follows.
In a case where the reception SINR at the mobile station keeps decreasing, the transmission power control operation based on the transmission power control bits generated from the mobile station is not properly performed at the base station. One of the reasons of this is that the reception quality of the transmission power control bits transmitted from the mobile station through the uplink is not adequate at the base station, and, as a result, a transmission power control operation based on different values from the values represented by the transmission power control bits generated from the mobile station is performed at the base station. The cause of this problem resides in that the transmission power control operation at the mobile station transmitting transmission power control bits is not necessarily performed in such a manner as to maintain the reception quality of the transmission power control bits at a predetermined quality level at the base station.
In the above situation, the reception quality of the transmission power control bits transmitted from the base station deteriorates at the mobile station due to the decrease of the reception SINR at the mobile station. As a result, an improper transmission power control operation is performed at the mobile station. Due to the improper transmission power control operation, the reception quality of the transmission power control bits transmitted from the mobile station deteriorates at the base station. Furthermore, due to the deterioration of the reception quality of the transmission power control bits at the base station, a transmission power control operation is not properly performed at the base station, resulting in a further decrease of the reception SINR at the mobile station. If this situation continues, the communication between the mobile station and the base station might be terminated.
Also in a conventional CDMA mobile communication system, a base station and a mobile station perform transmission and reception of various types of information (such as the information as to inherent spread codes to be used, and the information as to the signal format of each individual channel) through a common control channel, before a start of information data transmission between the base station and the mobile station. Transmission and reception of signals in a predetermined format are then performed through individual channels determined based on the above various types of information, so as to establish signal synchronization between the two stations. The process of establishing synchronization may be carried out through the procedures shown in FIG. 21.
As shown in FIG. 21, a base station 200 starts transmission of signals in a predetermined format (downlink transmission) ({circle around (1)}). The signals in the downlink transmission contain transmission power control bits (transmission power control information representing a power increase or a power decrease) that exhibit a predetermine pattern. A mobile station 100 performs synchronization initiation for the signals from the base station 200 ({circle around (2)}). Through the synchronization initiation, synchronization is established (downlink synchronization establishment) ({circle around (3)}). After confirming the establishment of synchronization, the mobile station 100 starts transmission of signals in a predetermined format (uplink transmission) ({circle around (4)}), while performing a transmission power control operation based on the transmission power control bits that exhibit the predetermined pattern.
The base station 200 performs synchronization initiation for signals transmitted from the mobile station 100 ({circle around (5)}). Through the synchronization initiation, synchronization is established at the base station 200 (uplink synchronization establishment) ({circle around (6)}). A certain period of time is required from the start of signal transmission ({circle around (1)}) from the base station 200 till the establishment of uplink synchronization ({circle around (6)}) at the base station 200.
In the above process of establishing synchronization, the transmission power control operation at the mobile station 100 may be performed as shown in FIG. 22.
Initially, the base station 200 needs to start downlink transmission while not receiving signals from the mobile station 100 (see {circle around (1)} of FIG. 21). Therefore, the base station 200 transmits transmission power control bits that represent a pattern of a constant power increase (a pattern only consisting of bits of “1”) as well as the signals. After confirming the establishment of downlink synchronization ({circle around (3)}) of FIG. 21), the mobile station 100 starts increasing the transmission power at time t1 for the uplink transmission start (equivalent to {circle around (4)} of FIG. 21). The mobile station 100 then gradually increases the transmission power from an initial value P0 determined from a transmission loss or the like, in accordance with the transmission power control bits (1, 1, 1, 1, . . . ).
The transmission power control cycle in accordance with the above transmission power control bits is much shorter than a time required for establishing synchronization between the mobile station 100 and the base station 200. Therefore, once the transmission power control value reaches the maximum value Pmax in the transmission power control operation in accordance with the above transmission power control bits, the transmission power is maintained at the maximum value Pmax.
In this manner, the mobile station 100 performs such a transmission power control operation as to increase the transmission power to the maximum value Pmax, while transmitting signals. After uplink synchronization is established at the base station 200 through the synchronization initiation of the signals (at time t2), the base station 200 measures the signal-to-interference plus noise power ratio (hereinafter referred to as the “reception SINR) with respect to each signal from the base station 100, regarding the reception SINR as the reception signal quality. Based on the difference between the reception SINR and a target SINR that is set independently of the reception SINR, the base station 200 generates transmission power control bits (each of which represents a power increase or a power decrease). The base station 200 then transmits the transmission power control bits to the mobile station 100 (downlink transmission).
At the time when uplink synchronization is established at the base station 200 (at time t2), the mobile station 100 is performing such a transmission power control operation as to increase the transmission power to the maximum value Pmax. Because of this, the reception SINR from the mobile station 100 is much higher than the target SINR (excessive quality). Immediately after the uplink synchronization is established, transmission power control bits “0” representing a continuous power decrease are normally generated.
After the uplink synchronization is established at the base station 200 (after time t2), the mobile station 100 transmits signals (uplink transmission) while performing a transmission power control operation based on the transmission power control bits generated from the base station 200 in the above manner (closed-loop control). As a result, the transmission power at the mobile station 100 is controlled so that the reception SINR at the base station 200 can be maintained at the target SINR. In this situation, the mobile station 100 starts transmitting signals including information data in a predetermined timing.
In the above transmission power control operation at the mobile station 100 before information data transmission is started between the base station 200 and the mobile station 100, the mobile station 100 transmits signals while controlling the transmission power to increase rapidly in accordance with the transmission power control bits (1, 1, 1, 1, . . . ) representing a continuous power increase transmitted from the base station 200, after downlink synchronization with each signal from the base station 200 is established at the mobile station 100 (after time t1 in FIG. 22). By this transmission power control operation, the reception quality of signals from the mobile station 100 is rapidly improved at the base station 200. Thus, the downlink synchronization at the base station 200 can be established in an earlier stage.
In the signal transmission and reception process between the base station 200 and the mobile station 100, however, the transmission power at the mobile station 100 should only be maintained at such a transmission power value that the reception signal quality (the reception SINR, for example) can be maintained at the target quality (the target SINR, for example) (see the transmission power value after time t2 in FIG. 22). If the transmission power value at the mobile station 100 is increased to the maximum value Pmax or increased to a value close to the maximum value Pmax despite the transmission power at the mobile station 100 being controlled in such a manner as to only maintain the reception signal quality at the target quality at the base station 200, the transmission power at the mobile station 100 temporarily becomes excessive at the time immediately before information data transmission is started between the base station 200 and the mobile station 100. This results in unnecessary power consumption through the uplink, and unnecessary radio resource consumption at the base station 200.